The advantages with lignin separation from black liquor is already described in WO 2006/031175 and WO2006/038863. These patents disclose the novel process LignoBoost™ that is now sold by Metso, and wherein WO 2006/031175 disclose the basic two stage acidic wash process and WO2006/038863 disclose an improvement of the process where sulphate or sulphate ions are added to the process.
The LignoBoost™ process was originally developed for production of an alternative bio-fuel, where main focus was at reduction of residual metal content, especially sodium, as the residual metal content may corrode boiler or burners using the lignin fuel. In this objective to reduce residual metal content it was shown that it was very important to maintain the process at the acidic side, and leaching and washing of lignin was kept at pH between 2-3 avoiding redeposition of metals, especially sodium, on the lignin.
An important aspect of the process is that the required charge of chemicals/acidifiers for the acidification and leaching of metals and subsequent washing may be high. If this is the case the cost of fresh chemicals is a large part of the operational cost and the commercial viability of the process is lower.
These problems could be reduced, if the process is optimized for minimum requirement for charges of fresh chemicals or acidifiers, keeping operational costs down and thus making the lignin product commercially sound.
Another consideration is to minimize the acidic waste flows from the process as conventional recovery of spent chemicals may be impeded if volumes of acidic waste flows increase in relation to the alkaline bulk volume of black liquor being recovered. Most often must acidic waste flows be handled separately if volumes are excessive which increase investments costs in recovery systems as well as operational costs of the mill.
Acidifiers in form of mill generated waste flows is thus preferable as it may both solve a waste disposal problem and lessen environmental impact as well as such usage would decrease costs for new chemicals. As the precipitation of lignin requires acidification of alkaline black liquor flows, much of the total amount of acidifier is used to decrease the pH level down to the point of where lignin starts to precipitate. The first stage reaching this pH level typically reduce the pH level from about pH 13 in the original black liquor down to a pH level about 9.5-11.5.
The LignoBoost™ process produce a lignin product which if used as fuel is classified as a “green” fuel as being based upon recovered fuel. The idea with classification of “green” fuels is based upon the concept not to increase the carbon dioxide footprint, i.e. the emissions, by burning fossil fuels. The most promising acids for this process is carbon dioxide for at least initial precipitation of the lignin, and then using sulfuric acid (H2SO4) for washing and leaching out metals from the lignin. The sulfuric acid could be added as a fresh sulfuric acid from a chemical supplier, or as preferred using so called “spent acid” from a chlorine dioxide generator often used at a pulp mill. The latter usage of this spent acid already at hand in most mill sites further emphasize that the lignin product is considered as a “green” fuel. However, interest in lignin as a base product for further usage has caught interest and in some applications further requirements on the lignin product is emphasized.
In U.S. Pat. No. 4,891,070 is disclosed a method for producing an improved aqueous printable ink composition from lignin. In this process extracting the lignin from the black liquor it is essential that the pH is not allowed to drop below pH 5 and adding an organic amine forming a lignin amine salt solution. In U.S. Pat. No. 4,891,070 is also disclosed other various methods of recovery, purification, and modification of lignin by-products as disclosed in U.S. Pat. Nos. 2,525,433; 2,680,113; 2,690,973; 3,094,515; 3,158,520; 3,503,762; 3,726,850; 3,769,272; 3,841,887; 4,001,202; 4,131,564; 4,184,845; 4,308,203; and 4,355,996 and concludes that generally the processes for obtaining purified lignin, includes pH decrease of black liquor to about 9,5 with subsequent acidification of the precipitate to a pH of about 1.5 to 4, followed by water washing to displace inorganic salts and impurities therein. However, no indication of hydrolysis of carbohydrates is shown. Thus, it could be concluded that extensive research work has been invested over several decades in finding the best process for obtaining a purified lignin product.
The original LignoBoost™ bio-fuel product produced a fuel with low residual levels of sodium and as it was used as a fuel no attention was drawn to the fact that the lignin fuel often had high levels of hemicelluloses, i.e. carbohydrates, as also hemicelluloses contributed to heat value of the fuel, even if the specific heat value of hemicelluloses is lower than pure lignin.
As the LignoBoost™ use black liquor from kraft pulping processes the hemicellulose content may vary considerably, and from worse case scenarios using cooking techniques like Lo-Solids Cooking could as much as 8-10% of the final lignin product correspond to hemicellulose/carbohydrate content.
Now is lignin considered also for usage in production of spun fibers, used when producing light weight construction details in automobiles and airplanes. But in this process the lignin purity must be higher and residual levels of hemicelluloses must be very low, well below 1 wt-%. In other applications are lignin also considered for production of chemicals and in these processes is higher lignin purity also a requirement.
It is known from handbooks in pulping processes that hemicelluloses could be removed from biomass in acidic prehydrolysis, which often was conducted at rather high temperatures. Prehydrolyse stages in pulping are typically conducted on the wood material at rather high temperatures, i.e. at about 170° C. in auto- or water hydrolysis and some 120-140° C. when wood material is slurried in dilute acid, all depending upon the established pH level (higher pH required higher temperature). In some applications have also prehydrolysis of wood chips been performed in strong acidic solutions (20-30% HCL at 40° C.), but this process led to extensive lignin destruction as well as alpha cellulose losses. Thus, if hemicellulose is to be removed selectively has always diluted acids been used. The dissolved hemicelluloses may also be further degraded at acidic conditions.
However, lignin is also known to decompose to solvable lignin if subjected to heat treatment at about 190° C. or higher, so the problem to reduce hemicelluloses content in a lignin product is not that obvious while maintaining the lignin yield high as well as reducing consumption of acidifiers and keeping acidic waste flows low. A major concern when using the LignoBoost™ process has been the filterability of the lignin throughout the process, and heating of lignin is well known to cause softening of the lignin and that negatively affects filterability. So, solving the problem with carbohydrates in lignin is not that obvious as lignin yield should be kept high while carbohydrate content should be kept low.